1. Field of the Invention
This invention provides a novel mounting package for RF transistors including a novel thermally conductive electrically insulating mounting pad, and relates to the semiconductor packaging industry.
2. Description of Related Art
The use of semiconductor devices for RF (radio or high frequency) applications has increased dramatically as heat dissipation, size, dependability, and other characteristics have made them particularly suited for many applications. Semiconductor transistors are very small and very difficult to use; therefore, they are generally incorporated into packages. Radio frequency packages are used for holding semiconductor components, particularly transistors, and for providing readily available terminals for connection to other components.
In designing such packages, the tendency of semiconductors to generate heat must be considered. To this end, the semiconductor is frequently mounted on a thermally conductive pad to act as a heat sink and dissipate heat generated by the transistor. However, semiconductors are sensitive to electrical energy, and therefore the thermally conductive pad should also be electrically insulating. Hence, a thermally conducting, electrically insulating material is used for the mounting pad. Commonly, this material is beryllia (beryllium oxide--BeO), although alumina (aluminum oxide--Al.sub.2 O.sub.3) has occasionally been used.
Beryllia (BeO) has better thermal conductivity than Al.sub.2 O.sub.3, and is therefore more commonly used. However, care must be taken in handling and processing BeO. BeO is highly toxic. It can be hazardous to the human respiratory system when in powder form. Therefore, care must be exercised if the BeO pad is to be machined or ground. Proper equipment and safeguards are needed to insure that the BeO is handled safely and that improper contact with humans is not made.
Another disadvantage of the prior art packages is that when BeO is used, a "thin" pad of BeO is employed. This pad is attached to the heat sink through the use of a preform. The preform is generally made of gold and tin, or silver and copper. The preform is placed between the mounting pad and the heat sink and the structure is heated. This melts the preform and brazes the pad to the heat sink. This creates two interfaces: pad-to-preform, and preform-to-heat sink. These interfaces decrease the thermal conductivity of the connection. Further, BeO has a high thermal conductivity up to about 200.degree. C. to 250.degree. C., but its performance decreases with increasing temperature.
The BeO pad used is generally about 0.040"to 0.060"thick. Such thickness is required since the mechanical strength of BeO is low. If the pad is not sufficiently thick, the pad may crack or split during attachment, or later upon heating from use.